Interface advance control in secondary recovery program by use of gradient barrier

ABSTRACT

The advance of the interface between driving and driven fluids in a secondary recovery operation toward a production well is delayed by the imposition of a gradient barrier of produced hydrocarbon fluids injected into the formation via a control well in line between an injection well and a production well, the recirculation of the formation hydrocarbon fluids providing a dynamic barrier.

United States Patent Inventor Donald L. Hoyt Houston, Tex. 786,568

Dec. 24, 1968 July 20, 1971 Texaco Inc. New York, N.Y.

Appl. No Filed Patented Assignee INTERFACE ADVANCE CONTROL IN SECONDARYRECOVERY PROGRAM BY USE OF GRADIENT BARRIER 23 Claims, 12 Drawing Figs.

US. Cl 166/245, 166/263, 166/266, [66/268 Int. Cl E211) 43/20, E21b43/22 Field of Search 166/245, 263, 268, 266

[56] References Cited UNITED STATES PATENTS 3.135.325 6/1964 Parker166/266 Re. 24.873 9/1960 Lindauer 166/268 3,074,481 1/1963 Habermann166/245 3,109,487 11/1963 Hoyt 166/245 3,215,198 11/1965 Willman 166/263Primary Examiner-Ian A. Calvert Attorneys-16 E. Kavanagh and Thomas H.Whaley ABSTRACT: The advance of the interface between driving and drivenfluids in a secondary recovery operation toward a production well isdelayed by the imposition of a gradient barrier of produced hydrocarbonfluids injected into the formation via a control well in line between aninjection well and a production well, the recirculation of the formationhydrocarbon fluids providing a dynamic barrier.

PATENTED M2!) B7! SHEET 2 OF 2 5 I 5 pg INTERFACE ADVANCE CONTROL INSECONDARY RECOVERY PROGRAM BY lUSlE OlF GRADIENT BARRIER FIELD OF THEINVENTION This invention relates generally to the production ofhydrocarbons from underground hydrocarbon-bearing formations, and moreparticularly, to a method for increasing the efficiency of theproduction of hydrocarbons therefrom.

DESCRIPTION OF THE PRIOR ART In the production of hydrocarbons frompermeable underground hydrocarbon-bearing formations, it is customary todrill one or more boreholes or wells into the hydrocarbonbearingformation and produce hydrocarbons, such as oil, through designatedproduction wells, either by the natural formation pressure or by pumpingthe wells. Sooner or later, the

flow of hydrocarbons diminishes and/or ceases, even though substantialquantities of hydrocarbons are still present in the undergroundformations.

Thus, secondary recovery programs are now an essential part of theoverall planning for virtually every oil and gas condensate reservoir inunderground hydrocarbon-bearing formations. In general, this involvesinjecting an extraneous fluid, such as water or gas, into the reservoirzone to drive formation fluids including hydrocarbons toward productionwells by the process frequently referred to as flooding." Usually, thisflooding is accomplished by injecting through wells drilled in ageometric pattern, the most common pattern being the fivespot.

When the driving fluid from the injection well reaches the productionwells of a five-spot pattern, the areal sweep is about 71 percent. Bycontinuing production considerably past breakthrough, it is possible toproduce much of the remaining unswept portion. It would be a greateconomic benefit to be able to achieve a sweep of 100 percent of thehydrocarbonbearing formation. It would be an even greater benefit to beable to achieve it at breakthrough, so that it would not be necessary toproduce large quantities of injected driving fluid.

It is understood that the failure of the driving flood in secondaryrecovery operations to contact or sweep all the hydrocarbon area is dueto the development of a cusp at the interface between the driving anddriven fluids, which advances toward the production well. If otherportions of the interface could be made to keep up, or if the cuspformation were delayed, a more complete areal sweep would be possible.In the commonly assigned U.S. Pat. No. 3,393,735, issued to A. F.Altamira et al. on July 23, 1968 for Interface Advance Control inPattern Flood by Use of Control Wells, there is disclosed how anincreased amount of hydrocarbons is produced and recovered from anunderground hydrocarbon-bearing formation by employing at least threewells, penetrating such a for' mation, which wells are in-line, toproduce hydrocarbons from the formation via two of these wells includingthe middle well, as disclosed in the commonly assigned U.S. Pat. No.3,109,487, issued to Donald L. l-loyt on Nov. 5, 1963 for PetroleumProduction by Secondary Recovery. in both these cited patents, there isdisclosed how a production control well is positioned between theinjection well and the production well and is kept on production afterthe injected fluid reached it. In this manner, the cusp is pinned" downat the control well and while the area swept out by the injection fluidbefore breakthrough at the outer production well is increased, there isan unwanted handling of considerable quantities of injected fluid at thecontrol well.

Another aspect to increase the sweep is disclosed in the commonlyassigned U.S. Pat. No. 3,393,734, issued to D. L. Hoyt et al. on July23, 1968 and involves the retardation of the development of the cusptoward a production well. The method of achieving more uniform advanceis to control the flow gradients so that the interface is spread out.This can be done either by choosing a particular geometry of well posi'tions or by adjusting the relative production rates so that the velocityof advance is not predominantly in one direction. It can be done also byshifting the gradients frequently, in both direction and magnitude, thuspreventing any one section of an interface from advancing too far outofline.

SUMMARY OF THE INVENTION It is an overall object of the presentinvention to provide an improved secondary recovery procedure involvinginitially three wells in line as part of a well arrangement forexploiting a hydrocarbon-bearing formation, by changing the function ofthe wells at strategic times to gain maximum control of the flood front.

A three-well group is arranged in line so that an end well is completedfor injection and the remaining two wells are offset and completed forproduction. Flooding is initiated at the end well by injection of anextraneous driving fluid, such as water or gas, thereinto and proceedsuntil breakthrough of the flood front occurs at the closer of the offsetproduction wells, at which time injection via the end well to maintainflooding is suspended and the offset production wells are put on astandby basis, e.g., by being closed in. Then preferably, a small volumeor slug of an extraneous fluid is injected into the formation via thecloser production well, at which breakthrough occurred, to drive theflood front away from this well, injection at the end well andproduction from the other production well are resumed, while continuingto inject a portion of the produced formation hydrocarbon fluids intothe converted production well. Examples of the extraneous fluid includeproduced formation hydrocarbon fluids, which may be treated withthickeners to increase the viscosity thereof, butane and propane, allbeing miscible with the formation fluids.

The continuous injection into the converted well establishes a system ofpressure gradients which on one side of the well are directed oppositeto the pressure gradients associated with the driving fluid. A point ofequilibrium of forces is established wherever the components of pressuregradient directed away from the converted well are equal and opposite tothe components directed toward that well. The locus of all suchequilibrium points establishes a stable interface, typically teardropshaped, around the converted well. The shape and size of this interfacewill depend upon the interrelationship of many factors, primarily,geometry of well positions, relative permeabilities and viscosities, andwell rate distributions. Control of any of these factors can be usedthereby to enhance the effectiveness ofthe method.

Since the injected driving fluid cannot penetrate this gradient barrier,it must travel a roundabout and longer flow path to reach the finalproduction well, thereby delaying cusping into the end production welland allowing a longer period for the advance of the interface betweenthe driving fluid and the formation fluids before breakthrough. Whenproduction at the end well is continued after breakthrough, theconverted injection well is closed in and the continuing productionresults in recovery of the injected produced hydrocarbon fluids, alongwith remaining in place formation fluids.

Many variations of this basic procedure are possible, and some will bemore advantageous than others for particular geometries of wellpositions, and reservoir and fluid parameters. But they will havecertain things in common:

1. An intermediate well between an injection source and a productionwell either exists or is added.

2. At breakthrough into this intermediate well (or some time priorthereto), the injection is suspended and a volume of fluid, such as aportion of the produced fluid hydrocarbons, is injected into theintermediate well. This portion may be a percentage of the pore volumewithin the drainage radius of the well. A simple approximation of thedrainage radius would be the average of the half distances to thenearest wells. This injection may be done with or without simultaneousproduction from other wells.

3. Injection of driving fluid is resumed at the principal injectionwell, and injection of the barrier fluid is continued into theintermediate well at a rate equal to a percentage of the productionrates.

4. ,This continues, even though other existing wells may be captured" byinjected driving fluid and closed in, until breakthrough of theinjection fluid into the production well on the other side of theintermediate well.

5. At this time, or in some cases even before it, the injection ofbarrier fluid is stopped. Continued injection of driving fluid on oneside and continued production on the other will move the barrier fluidcompletely into the production well if recovery of this fluid is desired(as, for example, if the barrier fluid used comprises producedhydrocarbon fluids).

Other objects, advantages and features of this invention will becomeapparent from a consideration of the specification with reference to thefigures of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 disclosed four units of aninverted fivespot pattern;

FIG. la is illustrative of the interface advance in the form of a cusptoward a comer production well in one quadrant of such a five-spotpattern undergoing secondary recovery;

FIG. 2 disclosed one unit ofa ninewell diagonal pattern;

FIG. 2a, corresponding to FIG. la, illustrates cusp accentuation in onequadrant of a nine-well diagonal pattern unit, and FIG. 2b illustratesthe effect of continued production from a control well to retard theadvance of the interface toward a comer production well i.e. "pinning"the cusp, in one quadrant of a nine-well diagonal pattern undergoingsecondary recovery;

FIG. 3 disclosed one quadrant of a nine-well diagonal pattern,illustrating the retreat of the cusp at the control well between theinjection well and the corner production well resulting from injectionof a small volume of produced fluid hydrocarbons into a control well,with all other wells temporarily shut in;

FIGS. 40, 4b and 4c illustrate the changes in well functions inaccordance with the movement of the interface during the several phasesof the production program in a 13-well pattern undergoing secondaryrecovery; and

FIGS. a, 5b and 5c correspond to FIGS. 4a, 4b and 4c during theproduction phases applied to a seventeen well pattern undergoingsecondary recovery.

The objects of the invention are achieved by the use of control wells incombination with production wells to delay the breakthrough of injecteddriving fluid into the outermost production wells, typically in patternunits, by retarding the development of the usual cusp interface betweenthe formation and injected fluids.

The specification and the figures of the drawings schematically discloseand illustrate the practice and the advantages of the invention withwell patterns and areal sweep examples which are obtainable and havebeen observed both in secondary recovery operations and inpotentiometric model studies which simulate secondary recoveryoperations. The model studies indicate a sweep obtained in an idealreservoir, although the recovery from an actual sweep of a particularfield may be greater or less, depending on field parameters.

Throughout the figures of the drawings, the same symbols will bemaintained as follows: P P,and P represent respectively production wellsat the comers, along the sides, and the interior control wells of apattern or well arrangement; and, a solid circle indicates a productionwell, a crossed circle indicates a shut-in well, an arrowed open circleindicates an injection well, and an arrowed solid circle, a convertedinjection well. The diagonal x-x in FIGS. 2, 4a and 5a represents anaxis of an injection well and a pair ofoffset production wells.

Referring to FIG. 1, there is disclosed four units of an invertedfive-spot pattern wherein the corner wells of each pattern unit areproduction wells, while the inner central well is used for injection.

FIG. Ia illustrates the growth of the cusp in one quadrant of aninverted five-spot pattern unit, wherein the secondary flooding fluid isinjected into the central well and production is maintained at thecorner wells until breakthrough, to result in a sweep of approximately71 percent.

Referring to FIG. 2, there is disclosed a nine-well diagonal pattern,essentially the five-spot pattern with control wells positioned on thediagonals between the central injection well and the corner productionwells. The control wells should be spaced at least one-half the distancebetween the injection well and each corner production well, with thebest results obtained when such control wells are positioned betweenthreefourths and seven-eighths of the distance from the injection welltoward the corner production wells. With such a pattern, the inventiondisclosed in the cited patent to Hoyt can be-cmployed with success toincrease the sweep area over that mentioned for the basic five-spotpattern.

FIG. 2a illustrates cusp accentuation after breakthrough has occurred atan interior control well, P,, located midway between the centralinjection well and the corner production well, after which this well isclosed in and production initiated and maintained at the cornerproduction well until breakthrough thereat.

FIG. 2b illustrates the effect of a control well as it retards theadvance of the cusp. As disclosed in the above-cited patents to Hoyt andto Altamira et al., the advance of the cusp has been pinned," therebydelaying the advance and accentuation of the pointed cusp interface, asillustrated in FIG. 2a, to produce an oblate cusp interface, resultingin a greater sweep. The procedure employed in achieving such a sweeprequires providing the flooding or driving fluid to the centralinjection well and maintaining production, either concurrently or inturn, at both the control or interior wells and the corner productionwells until breakthrough of the interface occurs at the cornerproduction wells. The sweep in each instance is substantially the same,with differences in the amounts of injected fluid produced at theinterior control wells.

In FIG. 3, the invention illustrates the improvement provided by oneembodiment of this invention over the disclosures of the prior art inFIGS. 1, la, 2, 2a and 2b. With injection of the driving fluid via thecentral well of a nine-well diagonal pattern, production is maintainedvia the interior control well, P (or also via the corner productionwell, P until breakthrough thereat, to form the interface indicated atA,P,A in FIG. 3, to yield a sweep of 57 percent. The control well, P,,is located on the ,diagonal through the injection well and the cornerproduction well, P about three-fourths of the distance from the centralinjection well. If the production well at P, were closed in to avoidhandling any of the injection driving fluid and production wereinitiated (or continued) and maintained till breakthrough at the cornerwell, P the sweep (not indicated) would increase by 9 percent, for atotal sweep of 66 percent.

Instead, a volume of produced formation hydrocarbon fluids equal to apredetermined value, e.g. about 15 percent of a pattern unit volume, isinjected through the interior control wells, P,-, from which productionhas ceased, and the cusp driven back by the resulting injected bubble ofhydrocarbon fluids, as shown in section at C FIG. 3, and the interfacedistorted as indicated by the dashed outline at A A the point of thecusp being driven back from the control well, P while the flanks advancefrom the line A,P A,. The changes in the shapes of the cusp have beenexaggerated for purposes of clarity.

When production is initiated (or resumed) at the corner well, P and withabout 50 percent of produced formation fluids being injectedcontinuously into the formation via well P the cycling hydrocarbonfluids form a stable teardrop bubble, as indicated in section at C FIG.3. The envelope of this bubble represents the surface of equilibrium ofpressure gradient forces.

By the time the driving fluid gets around the gradient barrier toachieve breakthrough at the corner production well, P the sweep of theformation fluids has been increased 31 percent,

as indicated by the interface at 13 F 8 for a total sweep of 88 percent.

Finally, if production at P, is continued after breakthrough with thecontrol well P, closed in, maximum gradients are reestablished along theaxis of the wells, and virtually all the injected produced hydrocarbonfluids can be recovered quickly along with additional formation fluidsmiscible therewith, bringing total sweep to over 90 percent before anappreciable percentage ofinjected driving fluid is produced.

The sweep can be increased either by forming a larger initial bubble orusing a greater fraction of the produced fluid hydrocarbons for injectedback into the formation via the cow verted control well.

FIG, la discloses the basic nine-spot pattern modified by the additionof four interior control wells, which can be positioned on the diagonalsof the pattern for best advantage as indicated previously. it can bevisualized also as a four-unit fivespot pattern, wherein the injectionwells of the inverted fivespot pattern units have been converted toproduction wells, and the innermost production well of the foununitlive'spot pattern has been converted into an injection well. li t ithsuch a conversion, the positions of the control wells have beenpredetermined and may not be situated for best effect.

As illustrated in one quadrant ofthc pattern, the first phase of theproduction method requires injecting driving fluid via the central welland production initiated and maintained at the remaining 12 wells of thepattern until breakthrough is achieved at the four interior controlwells, as shown in FIG.

4a, the clear area being the sweep and the right diagonals indicatingthe unswept in-place fluids. Then these interior production wells areconverted to injection wells for receiving produced formation fluidhydrocarbons while production is continued from the corner wells P andthe side wells P, until breakthrough thereat, as illustrated in FlG.ll), the left diagonals in the teardrop-shape section indicating thereturned fluid hydrocarbons. As indicated in H6. ts, the four side wellsare closed in, production is continued at the corner production wells1P," until breakthrough occurs thereat. By the illustrated phases ofthis method when applied to the nine-spot pattern as modified by theaddition of four interior control wells, a sweep of approximately 87percent follows. if production after breakthrough at the corner wells iscontinued, the teardrop bubble can be recovered in approximately 8percent of the time required for the third phase, bringing sweep to 93percent, leaving slivers of unswept areas adjacent the corner productionwells.

FIG. 5a discloses a 17-spot pattern which is formed by drilling a singleinjection well in a center of a 4X4 well square. In this l7-spotpattern, there are four corner production wells, two producing sidewells on each side of the 4X4 well square, and four interior controlwells located on the diagonals of the pattern and positioned between thecentral injection well and the corner production wells.

in FIGS. 5n, db and 5c, there are illustrated in one quadrant of thepattern, using the same symbolism as in M63. do, ll; and ile, threesteps or phases of the production method as applied to the l7-spotpattern. lit the first phase, illustrated in FIG. 5a, with injectionmaintained at the central injection well, production is initiated andmaintained at the four interior control wells, the eight side wells andeach ofthe corner wells until breakthrough is achieved at the controlwells. in the second phase, as illustrated in PM}. 5b, while productionis maintained at the corner production wells and the side wells, theinterior control wells are converted from production to injection ofproduced formation fluid hydrocarbons until breakthrough occurs at theside wells. Thereupon, the side wells P, are closed in, while productionis maintained until breakthrough at the corner production wells, lPinjection of the driving fluid is continued at the central well andinjection of produced fluid hydrocarbons is continued also via theinterior control wells, P interface positions at breakthrough into thecorner well, P are indicated in H6. fie. Again the favorable position ofthe injected fluid hydrocarbons is sweep before interface breakthrough.Clearly, the wider the barrier can be made, the better will be thesweep.

Factors which will cause such a barrier to be wide are: A. higherviscosity of the control well fluid than of the driving lluid;

B. high control well injection rates as a percentage of production;

C. production from side wells during the control well injection phase;

D. dual control wells straddling the axis through the injection andproduction wells to form a wider bubble, depending on the spacingbetween the straddle wells, such spacing being in the range of 0.1 to0.2 of the distance between the injection and production wells, (notshown in the drawing for purposes ofclarity).

Any pattern and/or rate distribution which retards the development, orthe advance, of a cusp towards production wells will increase the sweepof a field. Two principal means of doing this have been cited above,viz. (a) pinning" down the cusp by locating production wells between theinjection source and the outer production wells, and keeping such inner(or control) wells on production after breakthrough; and (b) spreading"out the cusp by pulling the front toward side wells until breakthroughthereat before allowing the interface to proceed toward the cornerproduction wells ofa pattern unit.

lillerein has been disclosed another method of delaying the advance ofthe interface in the form of a cusp toward an outer production well bylocating a dynamic barrier of produced fluid hydrocarbons between aninjection well and the outer production well.

Although emphasis has been placed in this disclosure on the practice ofthis invention as directed to a secondary recovery operation,particularly employing water or other similar aqueous fluid as theinjection displacement fluid, the advantages obtainable in the practiceof this invention are also realized in primary hydrocarbon productionoperations wherein the hydrocarbonbearing formation is under theinfluence of either a water or gas drive, or both a water and a gasdrive, and also in the instance of a secondary recovery operationwherein a gas, such as natural gas, is employed as the injection fluid.Moreover, the invention is applicable particularly to an arrangement ofa pair of production wells in line with an injection well under theinfluence of an active water drive.

l claim:

l. A method of producing formation fluids including hydrocarbons from anunderground hydrocarbon-bearing formation which comprises penetratingsaid formation with at least an injection well and an offset productionwell, injecting an extraneous driving fluid comprising natural gas intosaid formation via said injection well to displace fluids includinghydrocarbons in said formation toward said production well, producingsaid formation fluids including hydrocarbons from said formation viasaid production well until said extraneous driving fluid has reached apredetermined intermediate position therebetwcen, thereupon injectinginto said formation an extraneous fluid miscible with said formationfluids via an intermediate well between the injection and productionwells and continuing such injecting to form a dynamic barrier in saidformation therebetween, and maintaining producing fluids includinghydrocarbons from said formation via said offset production well whileinjecting extraneous driving fluid and that fluid miscible with saidformation fluids into said for-. mation via said injection andintermediate wells respectively.

2. In a method as defined in claim 1, said intermediate well being oneof a pair of wells straddling the diagonal between said injection andproduction wells and spaced apart from each other by a distance from 0.1to 0.2 of the distance between the last-mentioned wells.

3. in a method as defined in claim I, the extraneous fluid in-' jectedvia the intermediate well being selected from the group consisting ofbutane, propane and produced hydrocarbon fluids.

4. In a method as defined in claim 3, the intermediately injectedextraneous fluid being recovered from produced formation fluids via saidproduction well and returned to said formation via said intermediatewell until breakthrough of said extraneous driving fluid at saidproduction well.

5. A method of producing formation fluids including hydrocarbons from anunderground hydrocarbon-bearing for mation under the influence of anactive aquifer which comprises penetrating said formation with a pair ofproduction wells in line with the direction of advance of said aquifer,producing formation fluids including hydrocarbons displaced by saidaquifer until breakthrough of the interface between the formation fluidsand said aquifer at a production well, thereupon ceasing producingformation fluids thereat and injecting thereinto an extraneous fluid ofpredetermined volume, starting initially with a percentage of thereservoir volume and continuing with a percentage of the producedformation fluids from the other of said pair of production wells toprovide a dynamic gradient barrier at the interface where saidbreakthrough has occurred, and producing formation fluids includinghydrocarbons from said formation via the other of said pair ofproduction wells, while maintaining said gradient barrier.

6. In a method of producing formation fluids including hydrocarbons asdefined in claim 5, said extraneous fluid being selected from the groupconsisting of butane, propane and produced hydrocarbon fluids.

7. in a method of producing formation fluids including hydrocarbons asdefined in claim 6, said predetermined volume amounting to percent ofthe reservoir volume affected by the production well where saidbreakthrough has occurred.

8. In a method of producing formation fluids including hydrocarbons asdefined in claim 5, ceasing producing from said pair of wells whileinjecting said extraneous fluid.

9. A method of producing formation fluids including hydrocarbons from anunderground hydrocarbon-bearing formation which comprises penetratingsaid formation with at least three wells, a first well, a second welland a third well, said wells being substantially in line and the secondand third wells being on one side of said first well with said secondwell closer thereto, injecting an extraneous driving fluid into saidformation via said first well to displace fluids including hydrocarbonsin said formation toward said second and third wells, producing saidformation fluids including hydrocarbons from said formation via saidsecond well, recovering formation hydrocarbon fluids from producedformation fluids, ceasing producing said formation fluids and theninjecting some of the recovered formation hydrocarbon fluids into saidformation via said second well, and producing said formation fluidsincluding hydrocarbons from said formation via said third well andinjecting extraneous driving fluid and some recovered formationhydrocarbon fluids into said formation via the first and second wellsrespectively, the injecting of fluids into said formation via said firstand second wells continuing until breakthrough of said extraneousdriving fluid at said third well, thereupon ceasing injecting fluids viasaid second well while continuing producing said formation fluids viasaid third well. 10. in a method as defined in claim 9, injectingrecovered fonnation hydrocarbon fluids into said formation via saidsecond well prior to breakthrough of said extraneous driving fluidthereinto.

11. In a method as defined in claim 9, injecting recovered formationhydrocarbon fluids into said formation via said second well uponbreakthrough of said extraneous driving fluid thereinto.

12. In a method of producing formation fluids as defined in claim 9,concurrently initiating and maintaining producing said formation fluidsfrom said second and third wells.

13. In a method of producing formation fluids as defined in claim 9,initiating producing formation fluids from said third well afterstarting injecting of recovered formation hydrocarbon fluids into saidformation via said second well.

14. In a method of producing formation fluids as defined in claim 9,said three wells in line being part of a l3-well pattern, wherein thecentral well of said pattern is said first well and the remainingpattern walls are production wells arranged in equal numbers along thesides and on the diagonals of a quadrilateral, including said second andthird wells arranged therealong and eventually producing formationfluids therefrom.

15. In a method of producing formation fluids as defined in claim 14,simultaneously initiating producing said formation fluids via all ofsaid production wells.

16. in a method of producing formation as defined in claim 14, producingformation fluids via said wells located on the diagonals of said patternadjacent said injection well and continuing producing therefrom untilbreakthrough of said extraneous driving fluid occurs, thereuponconverting said aforementioned diagonal wells into injection wells andinjecting recovered formation hydrocarbon fluids into said formation viasuch converted wells and initiating and continuing producing from theside wells of said pattern until extraneous driving fluid breakthroughoccurs thereat, thereupon continuing injecting recovered formationhydrocarbon fluids into said formation via said converted wells andinitiating and maintaining producing said formation fluids via thecorner wells of said pattern until extraneous driving fluid breakthroughoccurs thereat.

17. in a method of producing fluids as defined in claim 9, said threewells in line being part of a 17-well pattern, the central well beingsaid first well and the remaining wells being production wells locatedin equal numbers along the sides and on the diagonals of a quadrilateralincluding said second and third weils arranged therealong, andeventually producing formation fluids therefrom.

18. In a method of producing fluids as defined in claim 17, continuinginjecting said extraneous driving fluid via said central well andproducing simultaneously from all of the remaining wells of the patternuntil said extraneous driving fluid breakthrough occurs at individualproduction wells on said diagonals, thereupon converting said individualproduction wells into injection wells and injecting recovered formationhydrocarbon fluids into said formation via the converted injection wellsand continuing producing said formation fluids via the remainingproduction wells until extraneous driving fluid breakthrough occursthereat.

19. in a method of producing fluids as defined in claim 17, continuinginjecting said extraneous driving fluid into said formation via saidcentral well and producing said formation fluids via said wells spacedon the diagonals of said pattern immediately adjacent the centralinjection well and continuing producing therefrom until extraneousdriving fluid breakthrough occurs at such production wells, thereuponconvetting such wells into injection wells and injecting recoveredformation hydrocarbon fluids into said formation via the convertedinjection wells and initiating and maintaining producing from the sidewells of said pattern until extraneous fluid breakthrough occursthereat, thereupon continuing injecting recovered formation hydrocarbonfluids into said formation via the converted injection wells andinitiating and maintaining producing at the corner production wellsuntil said extraneous driving fluid breakthrough occurs thereat.

20. In a method as defined in claim 9, said injecting of recoveredhydrocarbon fluids via said second well being of predetermined amountstarting with about 15 percent of the pore volume within the drainageradius of said second well and continuing with about 50 percent of theproduction of formation fluids from said third well to provide a dynamicbarrier therebetween.

2]. In a method as defined in claim 20, said recovered'formationhydrocarbon fluids being treated with thickeners to increase theviscosity thereof prior to injection into said formation via said secondwell.

22. In a method of producing formation fluids as defined in claim 9,said three wells in line being part of a nine-well diagonal patternwherein the central well of said pattern is said

2. In a method as defined in claim 1, said intermediate well being one of a pair of wells straddling the diagonal between said injection and production wells and spaced apart from each other by a distance from 0.1 to 0.2 of the distance between the last-mentioned wells.
 3. In a method as defined in claim 1, the extraneous fluid injected via the intermediate well being selected from the group consisting of butane, propane and produced hydrocarbon fluids.
 4. In a method as defined in claim 3, the intermediately injected extraneous fluid being recovered from produced formation fluids via said production well and returned to said formation via said intermediate well until breakthrough of said extraneous driving fluid at said production well.
 5. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation under the influence of an active aquifer which comprises penetrating said formation with a pair of production wells in line with the direction of advance of said aquifer, producing formation fluids including hydrocarbons displaced by said aquifer until breakthrough of the interface between the formation fluids and said aquifer at a production well, thereupon ceasing producing formation fluids thereat and injecting thereinto an extraneous fluid of predetermined volume, starting initially with a percentage of the reservoir volume and continuing with a percentage of the produced formation fluids from the other of said pair of production wells to provide a dynamic gradient barrier at the interface where said breakthrough has occurred, and producing formation fluids including hydrocarbons from said formation via the other of said pair of production wells, while maintaining said gradient barrier.
 6. In a method of producing formation fluids including hydrocarbons as defined in claim 5, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids.
 7. In a method of producing formation fluids including hydrocarbons as defined in claim 6, said predetermined volume amounting to 15 percent of the reservoir volume affected by the production well where said breakthrough has occurred.
 8. In a method of producing formation fluids including hydrocarbons as defined in claim 5, ceasing producing from said pair of wells while injecting said extraneous fluid.
 9. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells, a first well, a second well and a third well, said wells being substantially in line and the second and third wells being on one side of said first well with said second well closer thereto, injecting an extraneous driving fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and third wells, producing said formation fluids including hydrocarbons from said formation via said second well, recovering formation hydrocarbon fluids from produced formation fluids, ceasing producing said formation fluids and then injecting some of the recovered formation hydrocarbon fluids into said formation via said second well, and producing said formation fluids including hydrocarbons from said formation via said third well and injecting extraneous driving fluid and some recovered formation hydrocarbon fluids into said formation via the first and second wells respectively, the injecting of fluids into said formation via said first and second wells continuing until breakthrough of said extraneous driving fluid at said third well, thereupon ceasing injecting fluids via said second well while continuing producing said formation fluids via said third well.
 10. In a method as defined in claim 9, injecting recovered formation hydrocarbon fluids into said formation via said second well prior to breakthrough of said extraneous driving fluid thereinto.
 11. In a method as defined in claim 9, injecting recovered formation hydrocarbon fluids into said formation via said second well upon breakthrough of said extraneous driving fluid thereinto.
 12. In a method of producing formation fluids as defined in claim 9, concurrently initiating and maintaining producing said formation fluids from said second and third wells.
 13. In a method of producing formation fluids as defined in claim 9, initiating producing formation fluids from said third well after starting injecting of recovered formation hydrocarbon fluids into said formation via said second well.
 14. In a method of producing formation fluids as defined in claim 9, said three wells in line being part of a 13-well pattern, wherein the central well of said pattern is said first well and the remaining pattern walls are production wells arranged in equal numbers along the sides and on the diagonals of a quadrilateral, including said second and third wells arranged therealong and eventually producing formation fluids therefrom.
 15. In a method of producing formation fluids as defined in claim 14, simultaneously initiating producing said formation fluids via all of said production wells.
 16. In a method of producing formation as defined in claim 14, producing formation fluids via said wells located on the diagonals of said pattern adjacent said injection well and continuing producing therefrom until breakthrough of said exTraneous driving fluid occurs, thereupon converting said aforementioned diagonal wells into injection wells and injecting recovered formation hydrocarbon fluids into said formation via such converted wells and initiating and continuing producing from the side wells of said pattern until extraneous driving fluid breakthrough occurs thereat, thereupon continuing injecting recovered formation hydrocarbon fluids into said formation via said converted wells and initiating and maintaining producing said formation fluids via the corner wells of said pattern until extraneous driving fluid breakthrough occurs thereat.
 17. In a method of producing fluids as defined in claim 9, said three wells in line being part of a 17-well pattern, the central well being said first well and the remaining wells being production wells located in equal numbers along the sides and on the diagonals of a quadrilateral including said second and third wells arranged therealong, and eventually producing formation fluids therefrom.
 18. In a method of producing fluids as defined in claim 17, continuing injecting said extraneous driving fluid via said central well and producing simultaneously from all of the remaining wells of the pattern until said extraneous driving fluid breakthrough occurs at individual production wells on said diagonals, thereupon converting said individual production wells into injection wells and injecting recovered formation hydrocarbon fluids into said formation via the converted injection wells and continuing producing said formation fluids via the remaining production wells until extraneous driving fluid breakthrough occurs thereat.
 19. In a method of producing fluids as defined in claim 17, continuing injecting said extraneous driving fluid into said formation via said central well and producing said formation fluids via said wells spaced on the diagonals of said pattern immediately adjacent the central injection well and continuing producing therefrom until extraneous driving fluid breakthrough occurs at such production wells, thereupon converting such wells into injection wells and injecting recovered formation hydrocarbon fluids into said formation via the converted injection wells and initiating and maintaining producing from the side wells of said pattern until extraneous fluid breakthrough occurs thereat, thereupon continuing injecting recovered formation hydrocarbon fluids into said formation via the converted injection wells and initiating and maintaining producing at the corner production wells until said extraneous driving fluid breakthrough occurs thereat.
 20. In a method as defined in claim 9, said injecting of recovered hydrocarbon fluids via said second well being of predetermined amount starting with about 15 percent of the pore volume within the drainage radius of said second well and continuing with about 50 percent of the production of formation fluids from said third well to provide a dynamic barrier therebetween.
 21. In a method as defined in claim 20, said recovered formation hydrocarbon fluids being treated with thickeners to increase the viscosity thereof prior to injection into said formation via said second well.
 22. In a method of producing formation fluids as defined in claim 9, said three wells in line being part of a nine-well diagonal pattern wherein the central well of said pattern is said first well and the remaining pattern wells, including said second and third wells, are production wells arranged in equal numbers on the diagonals of a quadrilateral, and eventually producing said formation fluids via all of said production wells.
 23. In a method as defined in claim 22, said second well being spaced away from said first well by at least three-fourths 035946706 of the distance between said first well and said third well. 